On Mars, Signs of Water Don't Necessarily Mean Signs of Life

By KENNETH CHANG

Published: March 9, 2004

Following the news from Mars, people might think it was the story of scientists who cried ''Water!'' over and over.

Last week, scientists working with the NASA rovers announced that they had discovered water. Nothing that had flowed lately, but evidence that an area near the Martian Equator was soaked billions of years ago.

That was the latest in water news. In 2002, NASA's Mars Odyssey spacecraft found water at the planet's south pole. Two months ago, another announcement said the European Space Agency's Mars Express made a similar finding.

The search for water is driven by the hypothesis that Mars, which resembles an airless Antarctica, was a wet and warm place early in its history, amenable to life. Thus, not all water is equal. The Mars Odyssey and the Mars Express found ice, not liquid water. Those findings show that Mars possesses a lot of water, but not that it was ever habitable.

For the first time, evidence from the rover Opportunity last week showed minerals that require persistent liquid water to form.

''It's telling us water wasn't just frozen in the near-surface crust, but that it was moving around,'' said Dr. James W. Head III, a professor of geological sciences at Brown. ''This is such a major change in our thinking about exploration of Mars. Really, what we're talking about tips the scales.''

On Friday, scientists working on the mission reported that the Spirit, Opportunity's twin on Mars's other side, had also found signs of water in a rock. That water was most likely small amounts in the lava that formed the rock.

Despite all the findings, many planetary scientists are far from convinced that Mars was ever much different from the cold harsh place it is today.

''It's still very puzzling,'' said Dr. Michael H. Carr of the United States Geological Survey. ''It's certainly not proven that Mars was warm, that it was ever Earthlike.''

From telescopes on Earth, scientists have known about water on Mars for more than 30 years. Water in the Martian atmosphere absorbs specific colors of light, although the amount is minuscule. If all of it fell out of the sky, it would amount to one twenty-five-hundredth of an inch of rainfall across the surface.

NASA's Viking orbiters in the 1970's found ice at the North Pole, and photographs showed landscapes that appeared carved by water, networks of river channels, dried-up lakes and even what looks like shorelines of an ocean over the Northern Hemisphere. That led to speculation that volcanoes had disgorged a blanket of carbon dioxide, causing greenhouse warming that raised temperatures above freezing.

Initially, computer simulations indicated that the warming was plausible, but climate models on global warming are, on Mars as on Earth, an incomplete science. Further investigation showed that at high concentrations, carbon dioxide forms clouds that would reflect sunlight and stymie warming. Another complication is that the Sun, in the first billion years of the solar system, was 30 percent dimmer than it is today.

''All the climate modeling says it's very difficult,'' Dr. Carr said.

Other recent information from orbiting spacecraft also argues against widespread water, showing large amounts of olivine, a mineral that falls apart in warm, humid conditions, and almost no limestone or clays that form at the bottoms of lakes and seas.

''It's disquieting we're not seeing more evidence from orbit for water flowing,'' Dr. Carr said.

That has led to an alternative hypothesis of a cold wet Mars. The gravitational tug of war among Mars, Jupiter and the Sun cause Mars's axis to sometimes stand straight up and sometimes to tip at a 60-degree angle. When it is tipped, the Sun shines down on each pole for a good part of the year, evaporating the ice that might fall as snow near the Equator. The river valleys and lake beds could have formed from snow melted by volcanic heating, like the springs at Yellowstone, or by the weight of the snow itself, much like the liquid water at the bottom of glaciers.

''You don't necessarily have to make the whole planet warm to make liquid water,'' Dr. Carr said.

All the interest in water leads to the ultimate goal of finding out whether life ever arose on Mars. But scientists' crying ''Life!'' is also a familiar refrain. An experiment on the Viking landers that looked for life generated a positive result, although most theorize that was because of unusual nonliving chemistry.

In 1996, NASA scientists said they found evidence of life in a Martian meteorite that landed in Antarctica. That, too, remains unconvincing to most scientists.

And in the last few years, some people, including the author Arthur C. Clarke, say NASA has photographic proof. Sir Arthur says patterns seen near the Martian South Pole, which most scientists attribute to an interplay of ice and dust, are actually banyan trees.

Photo: Networks of valleys suggest that rivers once flowed on the surface of Mars and that the planet might have been much warmer than today. (Photo by NASA)